The interspecies hybrid embryos between the sand dollar, Dendraster excentricus, and sea urchin, Strongylocentrotus purpuratus, arrest during gastrulation, and fail to produce proteins of the paternal species, with the exception of histones. None-the-less, their polysomes contain mRNA characteristic of both species in normal complexities. These mRNAs are being examined to determine if they are properly adenylylated, and are being tested in vitro to determine if they can be translated into normal paternal proteins. The polysomes themselves are being examined for the presence of paternal nascent peptides to assess the normalcy of termination. A further study, by 2D slab gel electrophoresis, will be made of the parentage of the newly synthesized proteins in the hybrid embryos to determine if paternal ones are made at an early stage, and then quickly destroyed or if they are ultimately synthesized at stages later than have been examined to date. Some histones of paternal parentage are synthesized in the hybrids, and these have been used to explore the relative contributions of egg and newly transcribed messages to early development. An analysis of non-histone chromatin proteins is underway, to examine the evolutionary conservatism of this class of proteins which are believed to include those which regulate gene expression. A study of the molecular size of stored egg messages, and the conservation of their sequences through early development in normal sea urchins is being continued.